Firestone was a phosphine bearing rock found on Pern. Dragons and fire lizards could chew it to produce phosphorous gases which were ignited to burn Threadfall out of the sky. There were two types of firestone:
1. One (also known as "Flamestone") was highly unstable and would explode on contact with water.
2. The other was much more stable, but needed time to be digested by dragons before they could flame. This was the type that was originally found in Dragonsdawn and was used in all Pern novels following Dragon's Fire in Pern's chronology.
The chewed firestone had to be expelled from the body after it was used up - because the dragons couldn't digest it. This was done by regurgitation. The end product was a wet, smelly, sand-like substance. Presumably, fire-lizards simply waited until they felt the need to expel it. Dragons learned that they needed to do this after fighting Thread. A dragon that didn't know of this would later find that the need to expel the remains became more like an involuntary bodily function.
All dragons could chew firestone, except golden Queens, who would simply belch up a steaming brown-grey puddle. Green dragons would be sterilized by chewing firestone - something thought to be a precaution added by Kitti Ping Yung to prevent Pern from becoming overwhelmed with dragons.
It was thought that, since the gold queens could not use firestone, they would be less likely to fight Thread directly, and would be less likely to be put is harm's way, being the only breeding females. It is also suspected that it may have been a result of Kitti Ping's traditional beliefs that the mother should not be allowed to fight.
It was hard to handle. Water would make it explode.
It had yellow and gray crystals in it. It was also harder for dragons to hold onto than the less explosive type. Many dragons were killed by the gas that built up, leaving many dragonless men.
The Shunned were allowed to work in the firestone mines.